David Cameron has ruled out making a decision on a third Heathrow runway until the end of the year against fears of being taken to court, a move that effectively gags ministers who oppose expansion.

The Prime Minister told the House of Commons the Government's decision ran the risk of being halted by a judicial review if he signalled a preference before examining the Airports Commission report published today.

But London Mayor Boris Johnson and Tory Cabinet ministers oppose the move against fears of the impact on their London constituents.

At Prime Minister's Questions today, he was mocked by acting Labour leader Harriet Harman for being "bullied by Boris", claiming he was in a "holding pattern above Heathrow and Boris won't let him land".

But Mr Cameron said: "This very thorough report that landed on my desk yesterday afternoon is going to get properly studied. If you make some precipitous decision, rule out one particular option, you will likely make the decision impossible to achieve because of a judicial review."

This afternoon, the Prime Minister's official spokesman said "all ministers" should carefully consider the report - a signal they should not dissent.

Asked directly if ministers ought not to speak out against Heathrow, the Prime Minister's official spokeswoman said: "The Government's position is that we should consider this report carefully before setting out the next step. He's clear that all ministers should follow that process.

"The point was that the Prime Minister was making was that it's important that the Government is not seen to rush to judgement on this. The Prime Minister's view is that there is a clear process and that should be followed by government. That's what ministers should be doing."

No.10 said no decisions had been taken on the timing of the Government's final response other that the first indications would be "in the autumn".

Asked if ministers had decided if a Parliamentary vote was needed or simply a new hybrid bill for the new runway, the spokeswoman replied: "I can't clarify that at this stage".

His comments prompted confusion over whether Cabinet ministers opposed to Heathrow's expansion being green lit would still be able to speak freely.

The Telegraph reported today a letter from Cabinet Secretary Sir Jeremy Heywood suggesting collective responsibility apply to those with "specific Ministerial responsibilities" if they had a a constituency conflict.

After the Commons session, the Prime Minister's official spokesman could not be categoric over whether Mr Cameron's "no ifs, no buts" pledge remained.

He would only point to the Conservative Party manifesto being party policy, which has no explicit promise on Heathrow.

The Government also faces pressure from Labour, which indicated it was now four-square behind airport expansion despite backing off under Gordon Brown.

Ms Harman said Labour "will support him and there will be a majority in the House" if the Prime Minister "makes a swift decision on the Davies report".

She warned "backing off over airports" would risk "losing the opportunity for Britain to be at the heart of the global economy".

"It seems like he is in a holding pattern above Heathrow and Boris will not let him land," she added in the Commons.